The present invention relates generally to fastener plates, i.e., plates containing raised fastening elements such as, for example, nuts, studs, basket-like and other raised fastening elements and a method for attaching said elements to sheet metal to form said fastener plates. More specifically, the present invention relates to such a method wherein ultrasonic vibratory welding apparatus is used to weld these raised fastening elements to sheet metal in order to form an integral fastener plate.
Ultrasonic vibratory spot welding processes for joining together two or more similar or dissimilar materials have been used for a number of years. Until recently, however, such methods were limited to use on thermoplastics, non-woven fabrics and metals where weld strength and integrity were not particularly important. This limitation was due, in large measure, to the problems associated with the ultrasonic welding methods employed, most of which were in prototype stages. In those instances when weld strength and weld integrity were important, i.e., when joining together structural aircraft panels and the like, resistance spot welding procedures were used.
Ultrasonic spot welding procedures have recently demonstrated strong potential for improved sheet metal assembly at reduced cost when compared with resistance spot welding and adhesive bonding techniques. Early studies have indicated that welds effected using prototype ultrasonic welding equipment such as, for example, a Sonobond M-8000 ultrasonic spot welder, were superior to welds produced using conventional resistance spot welding procedures. These early trials indicated that for virtually any material combination, an ultrasonically produced spot weld has an ultimate yield strength of more than 2.5 times that of a weld produced using resistance spot welding equipment. Further tests indicated that ultrasonically produces spot welding can be accomplished with a 75% time and cost savings over conventional adhesive bonding techniques. Until now, however, ultrasonic spot welding for large structural metal parts was not possible in a production environment because of the numerous problems associated with the procedures.
Ultrasonic vibratory welding is a metallurgical joining technique which utilizes high frequency vibrations to disrupt the surface films and oxides and which, therefore, promotes interatomic diffusion and plastic flow between the surfaces in contact without any melting of the materials. Briefly stated, the ultrasonic welding process consists of clamping or otherwise securing together the workpieces under moderate pressure between the welding tip and a support anvil and then introducing high frequency vibratory energy into the pieces for a relatively short period of time, from a fraction of a second to a number of seconds. In many instances, the pieces to be welded are also adhesively bonded together by the insertion of an adhesive bonding agent between the juxtaposed pieces before welding which results in a high strength joint with superior static and fatigue properties.
One example of an ultrasonic spot welder particularly adapted for use on structural metal workpieces is the Sonobond Model M-8000 Ultrasonic Spot Welder marketed by the Sonobond Corporation of West Chester, Pa. This welder includes a transistorized, solid state frequency converter which raises standard 60 Hz electrical line frequency to 15-40 kHz and then amplifies the output. The high frequency electrical power travels through a lightweight cable to a transducer in the welding head where it is converted to vibratory power at the same frequency. The vibratory power is, thereupon, transmitted through an acoustic coupling system to the welding tip and then through the tip into and through the workpieces, with the vibratory energy effecting the weld.
The Sonobond M-8000 Ultrasonic Spot Welder includes a wedge-reed, transducer coupling system which transmits lateral vibrations of a perpendicular reed member attached to it so that the welding tip at the upper end of the reed executes shear vibrations on the surface of the workpieces. The transducer includes piezoelectric ceramic elements encased in a tension shell assembly and operates at a nominal frequency of 15 kHz. A solid state frequency converter with a transistorized hybrid junction amplifier power the welder. The converter operates at a nominal frequency of 15 kHz with a power output variable up to about 4000 RMS RF watts. The welder may be tuned to a precise operating frequency. The frequency converter includes a wide-band RF power measuring circuit which samples output power and detects forward power and load power based on the principle of bi-directional coupling in a transmission line. The signal is processed electronically to provide true RMS values which are selectively displayed on an LED panel meter as either the forward or load power. Forward power is the output of the frequency converter delivered to the transducer in the welding head while load power is the transducer drive power acoustically absorbed by the anvil. The difference between the two readings is the reflected power induced by the load impedance mismatch and is minimized during the welding operation by impedance matching techniques.
It is common practice in the assembly of structural sheet metal assemblies such as, for example, doors, panels and other removable parts, to secure such structural parts together using raised fastening elements such as, for example, screws, studs, nuts and the like. In order to facilitate such assembly operations, "fastening plates", i.e., plates containing raised fastening elements which are riveted, bonded or otherwise secured to a base are used for facilitating assembly. For example, such fastening plates are commonly used in air craft assembly for securing doors or panels to other structural parts. Heretofore, in order to attach such raised fastening elements to a base plate it was necessary to either bolt, rivet or otherwise bond the elements to the base plate. Resistance welding techniques were not readily adaptable for such applications. As can readily be appreciated, such procedures are labor intensive, time consuming, extremely costly and, oftentimes, result in structurally inferior load bearing elements.
It has been found that, due to the inherent characteristics of the ultrasonic vibratory welding equipment it is possible to weld such raised fastening elements to a base plate relatively simply and inexpensively, yet still produce a strong weld between the raised fastening elements and the base plate.
Against the foregoing background of the invention, it is a primary object of the present invention to provide a fastener plate containing raised fastening elements such as, for example, studs, nuts, bolts, baskets and the like, wherein the raised fastening elements are ultrasonically welded to the base plate.
It is another object of the present invention to provide a method for attaching the raised fastening elements to the base plate.
It is yet another object of the present invention to provide such a method which relies on ultrasonic vibratory welding techniques to effect such attachment.
It is still another object of the present invention to provide such a method which is readily adaptable for use in a production environment.
It is yet still another object of the present invention to provide such a method which results in a strong bond between the raised fastening elements and the base plate to permit the resultant fastener plate to be used in load bearing applications.